Role of CYP3A4 and CYP2B6 in the in vitro N-demethylation of meperidine

Abstract

Meperidine (MEP) is an opioid analgesic that exerts its chief pharmacological action in the brain. It is metabolized in the liver by N-demethylation to normeperidine, a potent CNS stimulant. We investigated the in vitro formation of normeperidine by pooled human liver microsomes (HLM) and recombinant human CYP450s. Normeperidine levels were measured by HPLC. Initial screening of 16 CYP isoforms (MEP=85 μM) showed CYP2B6>CYP2C19>CYP2C18>CYP3A4 as the enzymes with the highest MEP N-demethylase activity. Apparent kinetic parameters of MEP N-demethylase activity are described below.(See Table) Normeperidine formation in HLM was reduced by 42% in the presence of ketoconazole (CYP3A4 inhibitor, 2 μM). Treatment with anti-CYP2B and anti-2C inhibitory antibodies resulted in minor inhibition (<15%) of MEP N-demethylation in the liver. To further characterize the in vitro formation of normeperidine, future experiments will include correlation studies in HLM with probe substrates. Our studies suggest that (1) CYP3A4 (but not CYP3A5) has a dominant role in the hepatic metabolic clearance of MEP, and (2) CYP2B6 (a brain CYP) might contribute to normeperidine formation in the brain. Clinical Pharmacology & Therapeutics (2004) 75, P85–P85; doi: 10.1016/j.clpt.2003.11.325 Microsomes Km (μM) mean±SE) Vmax (pmol/min/pmol CYP) (mean±SE) Intrinsic clearance (μl/min/pmol CYP) HLM–high affinity phase 45.4 ± 12.8 124 ± 17 2.73 HLM–low affinity phase 1240 ± 185 1230 ± 60 0.99 CYP2B6 356 ± 34 138 ± 3 0.39 CYP2C19 71.4 ± 19.6 20.6 ± 1.1 0.29 CYP2C18 149 ± 47 17.0 ± 1.1 0.11 CYP3A4 355 ± 80 25.0 ± 1.3 0.07 CYP3A5 1260 ± 10 10.2 ± 0.3 0.01